Electric machines, such as generators or motors, comprise a stator and a rotor. To generate a magnetic field, the rotor comprises a coil of electrical conductors. An insulation surrounds the electrical conductors in order to insulate them electrically against windings of the coil arranged adjacently to them and against the environment.
Faults in the insulation of the rotor can lead to winding short-circuits, that is to short-circuits between windings of the coil arranged adjacently. The result of this is that a lower exciting current flows through the short-circuited windings of the coil than through those windings of the coil that are not-short-circuited, whereby the short-circuited windings have a lower temperature than the windings that are not short-circuited. This can lead to a non-homogeneous operating temperature of the rotor. The non-homogeneous operating temperature can lead to mechanical tensions within the rotor, and the tensions can lead to a deviation of the mass distribution from the rotational symmetry. The non-symmetrical mass distribution can lead to vibrations of the rotor during operation of the electric machine. In addition, the winding short-circuit leads to a weakening of the magnetic field, which must be compensated for by a higher exciting current. The higher exciting current disadvantageously leads to a reduction in the efficiency of the electric machine.
In operation of the electric machine, winding short-circuits are determined by means of an air-gap coil measuring method, in which the magnetic flux at locations between the rotor and the stator is measured by means of a coil. For this purpose it is necessary to identify a half-rotation of the rotor in the signal curve generated by means of the coil. Since the signal curve can be overlaid by a noise signal, the identification of the half-rotation of the rotor is, however, subject to error, so that the evaluation of the air-gap coil measuring method can lead to ambiguous results.